In a vehicle in which air pressure is used to operate a service brake, parking brake, clutch, suspension, or another auxiliary machine, a protection valve as disclosed in any of Patent Literatures 1 to 5 is used in order to enhance the reliability of the air system.
The conventional protection valves all include an input port for compressed air, output ports from which air is discharged, and safety valves corresponding to the output ports. The circuit configuration of a conventional protection valve is described below with reference to FIG. 13. FIG. 13 is a circuit diagram of a protection valve (which is hereinafter referred to simply as “valve”) 110 disclosed in Patent Literature 1.
In FIG. 13, the valve 110 includes an inlet 111 connected to a pressure source, an outlet 113 connected to a first main brake circuit 112, an outlet 115 connected to a second main brake circuit 114, an outlet 117 connected to a spring brake circuit 116, and an outlet 119 connected to an auxiliary machine circuit 118.
A first valve 120 is provided between the inlet 111 and the outlet 113. Similarly, a second valve 121 is provided between the inlet 111 and the outlet 115, a third valve 122 is provided between the secondary side (outlet) of the first valve 120 and the outlet 117, and a fourth valve 123 is provided between the secondary side (outlet) of the second valve 121 and the outlet 119. Check valves 132 and 133, which permit a flow only from an inlet to an outlet, are provided in the middle of the connection passages 130 and 131 connecting the third valve 122 and the fourth valve 123 to the first valve 120 and the second valve 121, respectively.
The valve 110 is provided with a first bypass line bypassing the first valve 120, and the first bypass line is provided with a check valve 125 which permits a flow only from an inlet to an outlet and a throttle valve 126. Similarly, the valve 110 is provided with a second bypass line bypassing the second valve 121, and the second bypass line is provided with a check valve 128 which permits a flow only from an inlet to an outlet and a throttle valve 129.
The inlet 111 and the outlet 117 for the spring brake circuit is communicated with each other via a passage 134, and a check valve 135 which permits a flow only from the outlet 117 for the spring brake circuit to the inlet 111 and a throttle valve 136 are provided in the passage 134.
According to the valve 110, if the first main brake circuit 112 fails and air leaks therefrom when the vehicle is left undriven for a whole day and night, for example, the air reserved in the air tank of the spring brake circuit 116 reaches the inlet 111 via the check valve 135 and the throttle valve 136 in the passage 134. The air is then discharged from the failed first main brake circuit 112 via the first throttle valve 126, the check valve 125, and the outlet 113.
In this case, the pressure in the spring brake circuit 116 becomes too low to supply air necessary to release the spring brake to the air chamber of the spring brake device. As a result, the vehicle cannot be started even when the driver performs a spring brake releasing operation. Accordingly, a safety problem can be eliminated because the spring brake cannot be released until the second main brake circuit 114 becomes functional after the start of the engine (start of the pressure source).